When I encounter Laura Benanti in the backstage green room of the Lyceum, she is sprawled face down on a sofa. An afternoon rehearsal has just ended, and there is an evening performance to come. Though she immediately comes to lively attention for our chat, Benanti admits that she is a little tired.

"One of the effects of having this gift of a role," she says, using a descriptive word related to the name of her In the Next Room character, Mrs. Givings, "is that it takes a lot out of you. Sarah [Ruhl] combines all the fractions of the female persona. You rarely get that in a play. You have to multi-task emotionally, which is something that women are used to doing in life but we're not used to seeing given full expression in theater or the movies."

"Mrs. Givings," Benanti continues, "leads completely with her emotions. Which means that I have to play them - quite a challenge! But with all these colorings there are also some very basic qualities to her. For example, she has a kind of innocence, which I love. This is a play about sexuality, but it's not just a play about people learning how to get off." Benanti elaborates: "We live in a world in which little kids see booty magazines at the corner bodega. Explicit sex has become the background noise of our lives. In that climate, intimacy has become the shocking thing."

I ask Benanti how the sexuality of Ruhl's play compares to that of the Broadway revival of Gypsy, for which the actress won a Tony award in 2008. "My character in Gypsy takes off her clothes, but that was not about intimacy but about power - about a woman learning to exert it in a world of men."

In the era of In the Next Room, the 1880s, middle-class women had fewer options. "A woman couldn't be anybody without a man," Benanti replies. "Which is why I think the play's ending is shocking not just in terms our current world but in terms of what we've come to expect from the expected resolutions in fairy tales or 19th-century novels."

Like everyone in the cast, Benanti was a little startled during early previews about the sometimes rollicking amount of laughter coming at them from the audience. I tell her she's lucky to have had some basic training for this - the Christopher Durang play she did off-Broadway earlier this year, Why Torture Is Wrong, and the People Who Love Them.

"Yes, that was helpful," Benanti agrees, "but in that one I was basically the straight man for the laughs caused by Kristine Nielsen's character. In Sarah's play, I'm getting laughs from MY lines: lines that are serious - about the pain I feel about not breast-feeding my baby, for example."

"At first, that laughter hurt my feelings," Benanti explained. "But I have to remember that laughter comes out of many feelings - surprise, unease, discomfort. It isn't just about finding something funny."

It also helps that Benanti herself has an extremely acute sense of wit. Where does that come from? "Well, I grew up around someone with a fantastic sense of fun," Benanti replies, referring tp her mother, Linda Wonneberger, who was not only Benanti's humor tutor but her vocal coach, a role she has to this day. "My mom taught me to appreciate the absurdity in everyday situations."

Such as? "I'd have to mention my backstage life during The Sound of Music." That's the Broadway revival, in 1998, when at age 18 Benanti was whisked almost overnight from doing plays at her small high school in New Jersey to playing one of Rodgers & Hammerstein's sisters and understudying Rebecca Luker in the lead role of Maria. "There were many nights," Benanti says, "when I would stand around backstage dressed as a nun, having serious conversations with other actors who were dressed as Nazis. If you can't see the humor in that, you're in the wrong business."

BRENDAN LEMON is the American theater critic for the Financial Timesand the editor of lemonwade.com.